This invention relates to a reciprocating ram pump of the kind disclosed in, for example, our prior British Pat. No. 677136. This kind of pump comprises a ram mounted for reciprocation in a cylinder having inlet and outlet ports for admission and discharge of the liquid which is to be pumped in response to the suction and delivery strokes of the ram. Reciprocation of the ram is effected by means of a hydraulically operable piston and cylinder device which is controlled by a changeover valve to effect reversal of the ram at the end of each stroke. This kind of pump is widely used for feeding slurries such as clay slip or sewage sludge to pressure type filters and may also be used in transferring liquid/slurries from one point to a remote point. Such a pump is hereinafter referred to as "a reciprocating ram pump of the kind specified".
In the past, the reversal of the ram at the ends of its suction and delivery strokes has been controlled mechanically by means of trip gear actuated by a crosshead which is movable with the ram, the trip gear being arranged to reverse a pilot valve which, in turn, controls the changeover valve for reversing the connections between the piston and cylinder device and a motor driven oil pump.
More recently, the mechanical trip gear has been replaced by electrical sensors, e.g. proximity switches, which are actuated when the ram reaches preset positions and a solenoid operated pilot valve is employed to operate the changeover valve in response to actuation of the electrical switches at each of the limiting positions of the ram.
One of the problems encountered with systems utilising pumps of the kind specified in conjunction with filter presses is the determination of the end point of the treatment cycle. From practical experience, the operator will, in general, be able to calculate at least approximately the volume of slurry that will need to be fed into the press before the desired filter cake consistency is approached. Another parameter which allows the operator to ascertain the approach of the end point is the flow rate into the filter. As the treatment cycle approaches completion, the pressure within the filter gradually increases with consequent reduction in the output of the ram pump.
Measures of the total volume delivered and instantaneous flow rate may be obtained by incorporating a suitable flow rate meter into the system but this would increase the capital costs of the system since reliable flow rate meters tend to be relatively expensive.